1. Field of the Invention
The present invention relates to a shadowing compensation device for a solar cell module and, more particularly, to a shadowing compensation device that outputs a compensation current to a shaded solar cell module of a solar cell array, thereby improving an output voltage and output power of the entire solar cell array.
2. Description of the Prior Art
A solar cell array is comprised of multiple solar cell modules connected in series. When setting up the solar cell array on buildings, the solar cell array may be installed on a roof or a top floor. However, the circumstances may have water tanks, parapet walls, roof ventilators, etc. around the solar cell array. It may result in a shadow, such that one or more solar cell modules will be shaded. Since the solar cell modules are mounted at designated positions, the specific solar cell module possibly being shaded may be known beforehand. In other words, some specific solar cell modules in the solar cell array may encounter the shadow problem.
When the solar cell module has been completely or partially shaded, the shaded solar cell module may reduce its output current. If a total output current of the whole solar cell array is controlled to be larger than the output current of the shaded solar cell module, the shaded solar cell module can no longer generate energy, and turns to be a load. Thus, the shaded solar cell module will overheat and eventually cause damage.
To prevent the shaded solar cell module from causing damage, each solar cell module will be connected with one or more inversed diodes in parallel during packaging processes of the solar cell module. When the solar cell module has been shaded, the diode is conducted to clamp a voltage across the shaded solar cell module to about 0 volt. Since the shaded solar cell module stops providing the output power, the solar cell array will decrease its total output voltage and the total output power. Multiple peak points will appear on the P-V (power-voltage) curve that shows relationships between the maximum output power (P) and the total output voltage (V) of the solar cell array, and will cause great complexity in tracking the maximum output power of the solar cell array.
With reference to FIG. 1, a conventional voltage balancing circuit for the solar cell array is proposed to deal with the problem of unequal voltage resulting from the shaded solar cell module. The voltage balancing circuit is connected to all solar cell modules PV1-PV4 for controlling their voltages to be equal. When any one of the solar cell modules is shaded, the shaded solar cell module can still sustain its output voltage and provide partial power. Thus, the total output power of the whole solar cell array remains regular.
For a solar cell array comprised of n number of solar cell modules, the voltage balancing circuit needs an inductor, n capacitors, (n+1)×2 diodes, and n×2 power switches. For example, the solar cell array shown in FIG. 1 has four solar cell modules PV1-PV4 (n=4), and therefore the voltage balancing circuit should have an inductor L, four capacitors C1-C4, ten diodes D1-D10, and eight power switches S1-S8. As the number of solar cell modules increases, the corresponding capacitors, power switches and diodes of the voltage balancing circuit will proportionally increase in number, which causes relatively expensive cost and complicated circuit connections. Because the power switches S1-S8 are alternately switching at high frequency when any one of the solar cell modules is shaded, the efficiency will deteriorate.
With reference to FIG. 2A, another conventional way for solving the problems of shadowing is shown. Four solar cell modules PV1-PV4 are connected in series as a solar cell array. Each solar cell module PV1-PV4 has two output terminals connected to a respective input terminal of a DC-DC power converter 41. Output terminals of the four DC-DC power converters 41 are connected in sequence and then further connected to input terminals of a power converter 42. Each DC-DC power converter 41 controls the output voltage of the respective solar cell module PV1-PV4. When any one of the solar cell modules PV1-PV4 is shaded, the shaded solar cell module can still provide partial output power to improve the total output power of the solar cell array.
With reference to FIG. 2B, each DC-DC power converter 41 needs two capacitors C1-C2, five power switches S1-S5, an inductor L and a diode D1. The capacitor C1 is parallel connected to the output of a solar cell module. The diode D1 is parallel connected to the output terminal of DC-DC power converter 41. If a solar cell array consists of n solar cell modules accompanied with DC-DC power converters, such large number of elements will cause the problems of high cost and complicated circuit connections.
The present invention thus develops a shadowing compensation device for a solar cell module to mitigate or solve the problems mentioned above.